14. Ischemia

Ischemia is, at its foundation, a mismatch between the rate at which oxygenated blood is delivered to a capillary bed and the rate at which the resident tissue consumes oxygen and substrate. The delivery side is governed by an Ohm's-law analogue in which flow equals the driving pressure divided by the resistance of the supplying vessels. For the brain, where this physiology is most exquisitely tuned and most clinically scrutinized, cerebral blood flow obeys $\mathrm{CBF} = \mathrm{CPP}/\mathrm{CVR}$, where cerebral perfusion pressure is mean arterial pressure minus intracranial (or jugular venous) pressure, $\mathrm{CPP} = \mathrm{MAP} - \mathrm{ICP}$, and cerebrovascular resistance is dominated by the tone of pre-capillary arterioles. Normal global CBF is approximately $50\,\mathrm{mL}\,/\,100\,\mathrm{g}\,/\,\mathrm{min}$, partitioned into roughly $80$ in gray matter and $20$ in white matter, a regional difference that explains why the cortical ribbon and deep gray nuclei are the first structures to declare ischemia on imaging. The delivered oxygen itself is set by the oxygen-delivery equation, $\mathrm{DO_2} = \mathrm{CO}\times C_aO_2$, with arterial oxygen content $C_aO_2 = (1.34\times \mathrm{Hb}\times S_aO_2) + (0.003\times P_aO_2)$; the overwhelming majority of oxygen is hemoglobin-bound, so anemia and desaturation lower the substrate ceiling independently of flow, and a tissue that is marginally perfused can be tipped into frank ischemia by a fall in hemoglobin or saturation. The Fick principle closes the loop: oxygen consumption equals delivery times the fraction extracted, $\mathrm{CMRO_2} = \mathrm{CBF}\times C_aO_2 \times \mathrm{OEF}$, where the oxygen extraction fraction is normally near $0.3{-}0.4$.

The defense of aerobic metabolism against falling perfusion pressure proceeds through a defined hierarchy. The first line is autoregulation: across a plateau of CPP from roughly $50$ to $150\,\mathrm{mmHg}$, arteriolar smooth muscle constricts or dilates — through myogenic, metabolic (CO$_2$, adenosine, $\mathrm{H^+}$, $\mathrm{K^+}$), and endothelial (nitric oxide) signaling, with flow–metabolism coupling matching regional CBF to neuronal activity — so that CBF is held nearly constant by adjusting CVR. As pressure falls toward the lower autoregulatory limit, the arterioles reach maximal dilation and cerebral blood volume rises; this is the first compensatory reserve and the physiological basis of the elevated CBV seen on perfusion imaging in still-viable tissue. When dilation is exhausted and CBF begins to fall, the second reserve engages: the oxygen extraction fraction rises toward unity, sustaining $\mathrm{CMRO_2}$ even as flow declines — the state of 'misery perfusion.' Only when extraction is maximal and flow continues to drop does delivery fall below the metabolic requirement and true ischemia begin. These two reserves — volume recruitment then extraction — are precisely what perfusion CT and MR attempt to render quantitatively as CBV, CBF, mean transit time (MTT, related by the central volume principle $\mathrm{CBF}=\mathrm{CBV}/\mathrm{MTT}$), and the time-to-maximum of the residue function (Tmax). Understanding this physiology is the prerequisite for everything that follows: the imaging of ischemia is, in essence, the imaging of where a tissue sits along this cascade of exhausted reserves, and the central diagnostic act is to distinguish tissue that has merely recruited its reserves (oligemia, salvageable penumbra) from tissue whose delivery has fallen below the structural threshold at which membranes fail.

Once delivery falls below consumption, injury unfolds as a stereotyped biochemical cascade whose stages map onto sharply defined flow thresholds and, critically, onto a point of reversibility that imaging and reperfusion therapy exist to exploit. Neurons consume the majority of their ATP maintaining the transmembrane ion gradients set by the Na$^+$/K$^+$-ATPase. When CBF falls below approximately $20\,\mathrm{mL}\,/\,100\,\mathrm{g}\,/\,\mathrm{min}$, oxidative phosphorylation can no longer regenerate ATP fast enough; protein synthesis halts and electrical activity is suppressed, producing the clinically silent but structurally intact state of the penumbra. This is the threshold of functional failure, and it is reversible: restore flow and the tissue resumes activity. As flow falls further, toward and below roughly $10{-}12\,\mathrm{mL}\,/\,100\,\mathrm{g}\,/\,\mathrm{min}$, ATP collapses, the ion pumps fail outright, and the gradients dissipate. Sodium and calcium pour into the cell down their electrochemical gradients, chloride and water follow osmotically, and the cell swells — cytotoxic edema. This shift of water from the extracellular into the intracellular compartment, followed by a small net increase in total tissue water as the failing membranes and the ischemic microvasculature progressively admit fluid, is the physical event that CT detects: because brain water is less attenuating than normal parenchyma, any net rise in tissue water content lowers attenuation by a small but clinically measurable amount. The threshold below which infarction becomes inevitable if flow is not restored — the structural threshold — is the boundary that defines the ischemic core.

The transition from reversible to irreversible injury is driven by a self-amplifying molecular program. Loss of the membrane potential triggers massive release of glutamate, whose action at NMDA and AMPA receptors opens further cation channels — excitotoxicity — flooding neurons with calcium. Intracellular calcium overload activates phospholipases, proteases (calpains), and endonucleases, generates reactive oxygen species, and opens the mitochondrial permeability transition pore, collapsing the mitochondrial membrane potential and committing the cell to death by necrosis (with apoptotic contributions, especially at the penumbral margin). The point of no return is therefore biochemical and time-dependent: it is the moment mitochondrial failure and calcium-activated catabolism become irreversible, and it arrives sooner in the core and later, or not at all, in the penumbra — which is why the penumbra is a time-dependent volume that the core consumes at a rate set by collateral flow. The temporal-density relationship is quantitatively predictable. Each 1% increase in tissue water lowers attenuation by approximately $2.5{-}2.6$ HU, and in middle cerebral artery infarction gray-matter attenuation falls at roughly $1.5{-}2.6\,\mathrm{HU}$ per hour over the first hours. The earliest CT signs follow directly: obscuration of the lentiform nucleus and loss of the insular ribbon (deep gray structures with high baseline flow and tenuous collateral supply), effacement of cortical sulci from cell swelling, and loss of the cortical gray–white interface. The expert reads these as the imaging transcription of a precise biochemical state, and the Alberta Stroke Program Early CT Score (ASPECTS) quantifies the spatial extent of this irreversible change to bound the salvageable tissue that therapy can rescue.

The CT manifestations of ischemia form a continuous temporal narrative from the hyperacute clot to the chronic cavity, and expert interpretation depends on reading the finding against the clock. In the hyperacute window (0–6 hours), non-contrast CT is frequently subtle, and its first sign is often vascular rather than parenchymal: the hyperdense artery sign, an intraluminal thrombus measuring approximately $50{-}90$ HU against the $35{-}45$ HU of flowing blood — a difference reflecting clot retraction and the high attenuation of densely packed erythrocytes and fibrin, formalized by an absolute threshold or a clot-to-contralateral ratio $>1.2$. Parenchymal signs in this window — insular ribbon loss, lentiform obscuration, sulcal effacement, and a focal hypoattenuation that respects an arterial territory — are the direct readout of cytotoxic edema and are best appreciated on a narrow stroke window (width $\sim30{-}40$ HU, level $\sim35{-}40$ HU), which expands the visual contrast across the small absolute HU decline. Over the acute-to-subacute interval (1–7 days), the hypoattenuation becomes confluent and unmistakable, mass effect peaks at 3–5 days as vasogenic edema is superimposed on the original cytotoxic swelling, and the territory may demonstrate the CT fogging phenomenon in the second week, when the infarct transiently regains near-normal attenuation as edema resolves and reparative hypercellularity and petechial hemorrhage raise density — a treacherous pseudo-normalization that can mask an established infarct unless contrast or prior studies disclose it. In the chronic phase (weeks onward), the tissue undergoes liquefactive necrosis and is resorbed, leaving encephalomalacia — a sharply marginated region of near-CSF attenuation ($\sim0{-}15$ HU) with ex-vacuo volume loss, gliotic margins, and compensatory dilation of the adjacent ventricle and sulci.

Functional CT perfusion converts this morphological narrative into a physiological map that separates the irreversibly infarcted core from the salvageable penumbra in the hyperacute window, when non-contrast CT is least informative. The parameter signatures follow directly from the perfusion physiology: the core shows severely reduced CBF and CBV (loss of both flow and the autoregulatory volume reserve), while the penumbra shows reduced CBF but preserved or elevated CBV (recruited reserve) together with prolonged MTT and Tmax. Contemporary thresholds, validated in the trials that govern late-window thrombectomy, operationalize the core as relative CBF $<30\%$ of the contralateral hemisphere and the critically hypoperfused tissue as $\mathrm{Tmax}>6\,\mathrm{s}$; the mismatch volume and mismatch ratio between these define the salvageable target. The table summarizes the discrimination.

The diagnostic reasoning is irreducibly Bayesian. A territorial wedge of hypoattenuation in an elderly patient with atrial fibrillation and sudden hemiparesis carries a high pre-test probability of arterial infarction; the same finding crossing vascular boundaries, centered on cortex with gyral swelling, in a febrile patient shifts the posterior toward encephalitis or a venous infarct. The principal mimics must be held in mind and actively excluded: hypoglycemia and status epilepticus produce diffusion and perfusion abnormalities that disrespect arterial territories; posterior reversible encephalopathy syndrome favors parieto-occipital white matter with preserved cortex; a low-grade neoplasm expands rather than effaces gyri and lacks an acute clinical tempo; and venous infarction — non-arterial in distribution, often hemorrhagic, with a dense dural sinus or cord sign — is the classic trap for the reader anchored to arterial patterns. The failure modes are equally important. Technically, partial-volume averaging at the skull base and beam-hardening from the posterior fossa obscure brainstem and temporal infarcts; motion and poor bolus timing corrupt perfusion maps and can manufacture spurious core or penumbra. Cognitively, satisfaction of search after identifying one lesion, anchoring on an initial normal CT, and inattention to the silent hyperdense vessel sign are the recurrent causes of missed hyperacute stroke. The disciplined reader therefore couples a narrow stroke window, systematic ASPECTS interrogation, and explicit consideration of the venous system and the metabolic mimics before committing to the diagnosis.

Restoring blood flow to ischemic tissue is the goal of every recanalization therapy, yet reperfusion is itself a source of injury, and the paradox that reflow can extend rather than only rescue damage is central to interpreting post-treatment imaging and to bounding therapeutic risk. The mechanisms are several and synergistic. Reintroduction of oxygen into tissue whose mitochondria are calcium-overloaded and whose antioxidant defenses are depleted produces an oxidative burst: xanthine oxidase, mitochondrial electron leak, and activated neutrophils generate superoxide, hydrogen peroxide, hydroxyl radicals, and peroxynitrite, which peroxidize membrane lipids, damage DNA, and further open the mitochondrial permeability transition pore. The abrupt normalization of pH on reperfusion — the 'pH paradox' — accelerates this pore opening and hypercontracture. Reperfusion also drives inflammation: adhesion-molecule upregulation, neutrophil and microglial activation, and cytokine release amplify tissue damage and, importantly, degrade the blood–brain barrier. Matrix metalloproteinases (notably MMP-9) and the inflammatory cascade digest the basal lamina and endothelial tight junctions, so that the barrier whose integrity normally excludes protein and contrast becomes leaky. Two consequences follow that are directly visible on CT. First, iodinated contrast staining of the infarct appears on immediate post-thrombectomy imaging as parenchymal hyperdensity; the diagnostic challenge is distinguishing benign contrast extravasation from hemorrhage, resolved by its tendency to wash out by 24 hours, attenuation often exceeding $\sim90$ HU, and lack of mass effect or characteristic blood-product evolution, with dual-energy CT iodine maps now able to separate the two definitively. Second, and more ominously, barrier breakdown permits hemorrhagic transformation.

Microvascular dysfunction compounds the problem through no-reflow: even after the occluding clot is removed, capillary beds may fail to reperfuse because of endothelial swelling, pericyte constriction, microthrombi, perivascular astrocyte foot-process edema, and leukocyte plugging — so that angiographically successful recanalization (TICI 2b–3) does not guarantee tissue-level reperfusion, and a subset of patients recanalize without clinical benefit (futile recanalization). Hemorrhagic transformation is graded by the ECASS classification, which carries direct prognostic and management weight; the distinction between petechial hemorrhagic infarction and confluent parenchymal hematoma is the difference between an incidental finding and a life-threatening, often symptomatic, deterioration.

The predictors of symptomatic hemorrhagic transformation are the levers of clinical decision-making: large baseline infarct core and low ASPECTS, severe hypoperfusion with poor collaterals, hyperglycemia, elevated blood pressure, delayed reperfusion, and thrombolytic exposure. These are precisely why core volume and collateral assessment on baseline CT/CTA/CTP are not academic measurements but determinants of whether thrombolysis or thrombectomy will help or harm. The most catastrophic reperfusion-adjacent outcome is malignant cerebral edema in large hemispheric infarction: progressive cytotoxic and vasogenic swelling produces midline shift, uncal and subfalcine herniation, and effacement of the basal cisterns, where CT findings of $>5\,\mathrm{mm}$ midline shift and large established infarct volume identify candidates for decompressive hemicraniectomy. The unifying lesson is that imaging after reperfusion must be read as a balance sheet: tissue salvaged against tissue injured by the very flow that was restored, with hemorrhage, contrast staining, no-reflow, and malignant edema as the four signatures the expert weighs to predict outcome and direct the next intervention.